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Adjustment to compensate for deck resistance

Discussion in 'Temperature Control' started by GaryParr, Jan 22, 2018.

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  1. GaryParr

    GaryParr Full Member

    Nov 25, 2012
    New England
    I completely agree that the deck resistance doesn't change. Where we are missing each other is understanding how the mod works. That black line is not some mythical thing I imagined. That represents the values the mod is calculating as the target values it needs to hit at given temperatures. The mod itself is treating the whole coil+deck as a single system, but it doesn't know that only part of that system is variable (the coil) and part (the deck) is constant. The mod just takes the base reading of 0.245 and uses the TCR to generate that black line. And that black line is wrong.

    Sorry to be stubborn about this, but honestly can you tell me where in my step-by-step of what's going on I missed something? I'll post it again here:
    1. Mod reads starting ohm of 0.245 (0.18 from coil, 0.065 from deck)
    2. I set my mod to a temp of 392F
    3. Mod uses TCR to calculate target resistance of 0.305858 (0.245 X 1.2484)
    4. Mod applies power until resistance reaches 0.305858
    If anything else is happening, please tell me. Again, I'm open to being proven wrong, I just need to know exactly where I went off the rails.
     
  2. Coastal Cowboy

    Coastal Cowboy This aggression will not stand, man! Verified Member ECF Veteran

    Supporting member
    Jun 13, 2013
    Alabama Gulf Coast
    Ignore the percentage change. For any TCR, the slope of the curve from any point to the next adjacent point is fixed. In a properly functioning device, calculated temperature does increase with resistance at an increasing rate. If the slopes of two curves are different, then they are using different TCR's.

    That's why I said the chart on the left was what was happening (assuming your device is working properly).

    If anything else is happening--if the slope of your curve is changing from left to right--increasing at an increasing rate--then the device doing the calculations is broken.

    If the slope of the curve is different from the one calculated by Steam Engine or any of the other 6,769 temperature curve calculators, then the device is broken.
     
  3. suprtrkr

    suprtrkr Vaping Master Verified Member ECF Veteran

    The math isn't hard to work out, provided you have stainless steel decks and know what grade of SS they are built of. The TCR for SS is known. To measure it, take a piece of copper wire and bridge the deck posts as short as you can, then measure the deck resistance. Then install the coil on the deck, and measure again. The coil measurement minus the deck measurement is the coil resistance, and A divided into B is the percentage belonging to the deck itself.

    I wouldn't have any idea how to program a mod to calculate this in real time, however. Maybe eScribe can do it, I dunno.

    Or you could just ignore it: switch to SS316 wire, if you haven't already, and ignore it as both the deck and the coil are similar materials and behave near identically.

    Edit: it's really minor, no matter what. Probably below the level where it's necessary to consider it. The deck does rise in temp, yes, and this does change its resistance. But it doesn't get anywhere near as hot as the coil. Ignore it, is my advice.
     
    • Like Like x 1
  4. GaryParr

    GaryParr Full Member

    Nov 25, 2012
    New England
    Ah Ha! I see where you are going wrong. The TCR and the base-line Ohm reading change the line slope. You don't have to take my word for it. I copied the following values straight off of steam-engine for 430 SS. You will notice that the table does not tell you what starting ohm you have... just what the resistance factor is for a given temperature. From there, you can calculate the resistance your coil should read at that temperature. That value differs depending on the starting resistance.

    [​IMG]

    As you can see, starting with 0.18 and 0.245 give you slightly different slopes. The TCR is constant. If you do some math you can see a there is a 0.000248 change in resistance per 1 C starting at 0.18 and a 0.000338 change in resistance per 1 C starting at 0.245.
     
  5. Coastal Cowboy

    Coastal Cowboy This aggression will not stand, man! Verified Member ECF Veteran

    Supporting member
    Jun 13, 2013
    Alabama Gulf Coast
    Awesomely awesome.

    Minor nit: The slope appears to change because the horizontal axis isn't uniformly scaled. When it is, TCR curveS look like this:

    [​IMG]

    (h/t emberswilde from VU)
     
  6. GaryParr

    GaryParr Full Member

    Nov 25, 2012
    New England
    From what I'm working through, it seems that in most instances, yes... the impact is not that large. However, the lower your coil resistance and the larger the built-in resistance of the deck, the larger variance you get between your desired temp and what the coil is actually hitting. For instance, a 0.15 Ohm coil with 0.025 Ohm of "other" resistance in the system will hit a temperature of 551F when the mod is set to 482F.
     
    • Agree Agree x 1
  7. suprtrkr

    suprtrkr Vaping Master Verified Member ECF Veteran

    No doubt you are correct. Still, if you want to get that picky about it, you're going to have to account for the other component resistance in the entire circuit. The 510 connector also heats up, and changes resistance. So does the motherboard in a regulated mod. Ditto battery contacts, lead wires, switch contacts, etc. How far you want to take that? Really, though, it'd take some empiricism (and a finely calibrated infrared thermometer) but you could eventually work out an exact TCR for any system combination and them just program it as a TCR figure. I am not certain why you'd want to (or feel any need for a .15Ω coil in a TC application, for that matter) but I don't use TC so maybe I'm out of touch. Heck, I finally found a good quality VW regulated mod that doesn't do TC, and ordered one less than an hour after I heard about it. Just yesterday that was :)
     
  8. Coastal Cowboy

    Coastal Cowboy This aggression will not stand, man! Verified Member ECF Veteran

    Supporting member
    Jun 13, 2013
    Alabama Gulf Coast
    You're getting a regulated mod???

    Wolves sleeping with sheep are next.

    :D
     
    • Like Like x 1
  9. suprtrkr

    suprtrkr Vaping Master Verified Member ECF Veteran

    I am:
    [​IMG]

    Nice chunk of gear.

    I do have regulateds, my friend. There are times when they're useful. Fishing trips, for example. Dripping is too much gear, and even squonkers don't do well in salt water. So I bang an RTA on a regulated and vape happy :)
     
    • Like Like x 2
  10. GaryParr

    GaryParr Full Member

    Nov 25, 2012
    New England
    I'm only as picky as my spare time allows, and I pretty much just ran out. So, I agree that a million little things will influence the accuracy of TC from mod to mod and tank to tank. However, what started me down this time suck path was a rather unusually high resistance from a single component. Based on the highly scientific method of anecdotal evidence, I'd be willing to wager that a not-insubstantial number of RTAs out there are adding a non-insignificant amount of resistance to a build. It may explain why someone feels 460F is a great vape on device A while 480F gives the same feeling on device B. Due to resistance differences, it could be the 460F device is over-powering and actually hitting at 480F. Or, it could be just any of the other little things out there that make tanks seem different.

    But, since I'm out of time to shovel into this hole I'll leave everyone with my sincere thanks for engaging in this discussion. Here is a spreadsheet I put together that will calculate everything we've been talking about. All you need to do is enter the starting resistance of the coil, the TCR for your build, and the resistance being added by the deck (or whatever else is adding resistance) and it tells you the correct TCR to use.

    https://1drv.ms/x/s!AkqRTy-kuNyHgbpiNH7Hp9xyjqZC9g
     
    • Like Like x 2
  11. GaryParr

    GaryParr Full Member

    Nov 25, 2012
    New England
    I found some more time on my hands.

    The spreadsheet is gone. Instead, I present the TCR Tweak Tool

    Hope someone finds it useful. Feedback is welcome.
     
    • Useful Useful x 1

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